When the Moon is opposite the Sun, for instance in Libra while he is in Aries, she is full. In 27½ days she returns to the same place among the stars, and this is a SIDEREAL MONTH. But the Sun meanwhile has moved into Taurus, and not until the Moon has reached Scorpio, opposite to him, will she be full again, and complete her SYNODIC MONTH (29½ days).] REVOLUTIONS OF SUN, MOON, AND PLANETS AS SEEN FROM THE EARTH. _Days. Hrs. Mins. Secs._ Tropical Solar Year: period from one vernal equinox to another, or from one summer solstice to the next, &c. 365 5 48 45·5 (Our civil year is based on this). Sidereal Year: period between two successive returns of the sun to any star on his path 365 6 9 8·9 The difference between these two kinds of year is due to Precession of the Equinoxes, _vide infra_, p. 23. _Days. Hrs. Mins. Secs._ Solar Day: period between two successive passages of the sun across the meridian (noon) — 24 0 0 Sidereal Day: period between two successive passages of a star across the meridian — 23 56 4 The difference between these two kinds of day is due to lag of the sun behind the stars, his daily motion westward being slightly retarded by his slow yearly motion eastward. _Days. Hrs. Mins. Secs._ Mean Synodic Month: period between two full moons 29 12 44 2·8 Mean Sidereal Month: period between two successive returns of moon to any star on her path 27 7 43 11·5 The difference between these two kinds of months is due to the fact that while the moon is making her revolution among the stars, the sun is also moving slowly on in the same direction. _Days. Hrs. Mins. Secs._ Mean Anomalistic Month: period between perigee and perigee (_vide infra_) 27 13 18 37·4 PLANETS:—Mean Synodic Revolution: period between two successive conjunctions with the sun, and mean zodiacal revolution: period of revolution round the zodiac. _Mean Synodic Revolution. Mean Zodiacal Revolution._ Mercury 116 Days[4] 1·0 Years Venus 584 ” 1·0 ” Mars 780 ” 1·88 ” Jupiter 399 ” 11·86 ” Saturn 348 ” 29·46 ”